Nonaqueous secondary batteries, such as lithium ion secondary batteries, have been widely used as power sources for portable electronic devices such as laptop computers, mobile phones, digital cameras, and camcorders. Further, these batteries are characterized by having high energy density, and thus their application to automobiles and the like has also been studied in recent years.
With the reduction in size and weight of portable electronic devices, the outer casing of a nonaqueous secondary battery has been simplified. As outer casings, battery cans formed of aluminum cans have been developed to replace battery cans made of stainless steel originally used. Further, soft pack outer casings formed of aluminum laminate packs have been developed nowadays.
In the case of a soft pack outer casing formed from an aluminum laminate, because the outer casing is soft, a gap may be formed between an electrode and a separator upon charging and discharging. This causes the reduction of cycle life. The maintenance of uniform adhesion between bonded parts of an electrode, a separator, or the like, is one of the important technical issues.
As techniques associated with adhesion, various techniques have been proposed for enhancing adhesion between electrodes and a separator. As one of such techniques, a technique of using a separator including a polyolefin microporous membrane, which is a conventional separator, and a porous layer made of a polyvinylidene fluoride resin (hereinafter sometimes referred to as “adhesive porous layer”) formed thereon has been proposed (see, e.g., Patent Document 1). When the separator is placed on an electrode and hot-pressed, the adhesive porous layer functions as an adhesive that joins the electrode and the separator well together. Therefore, the adhesive porous layer contributes to the improvement of the cycle life of a soft pack battery.
In relation to the separator including a polyolefin microporous membrane and an adhesive porous layer laminated thereon, for achieving both the ensuring of sufficient adhesion and ion permeability, a new technical proposal has been made focusing on the porous structure and thickness of the polyvinylidene fluoride resin layer.